The invention relates to radar countermeasures particularly for radars that lobe, i.e. employ their tracking scan, on receive only.
Tracking radars usually employ a conical scan of the antenna lobe in which the axis of symmetry of the lobe makes a slight angle to the antenna boresight and rotates at a constant rate thereabout generating the surface of a cone. If there is an angular error between the antenna boresight and the target, the return signal has an amplitude modulation at the rotational frequency of the lobe the amplitude of which represents the magnitude of the error and the phase of which relative to a predefined reference phase represents the direction of the error. The antenna aiming system of the tracking radar uses this information to reduce the angular error between boresight and target to zero.
In order to make countermeasures more difficult and to reduce the effect of spurious signals in general on the antenna aiming system of the radar, the signal acceptance of the aiming circuits is limited to a very narrow band of frequencies centered on the lobing frequency, i.e. the rotational frequency of the conical scan. Consequently, the signal radiated by a countermeasure device for the purpose of achieving angle deception of the radar must contain the lobing frequency in order to influence the antenna aiming. Where the radar both transmits and receives on the conically scanning lobe the lobing frequency can be determined at the target from the amplitude modulation of the incident radiation. In a LORO radar, however, only the receiving lobe scans, the transmitting lobe remaining fixed and symmetrical to the antenna boresight at all times. This type radar therefore transmits no information about its lobing frequency which creates the problem for the countermeasure device of determining this frequency and radiating a signal containing energy at or very near this frequency for a sufficient interval of time to break the tracking lock of the radar.